Machine to machine (M2M) communications is an emerging communication paradigm that has found applications in smart gird and vehicular communications.
Multi-hop relay networks have been identified as a network structure capable of accommodating the ever increasing demand of information exchange over a large number of nodes in M2M communications. In multi-hop relay networks, one or several intermediate relay nodes are employed to receive the information from the source node and then forward it to the other relay nodes or the destination node.
In some multi-hop relay networks, the relay nodes simply amplify the signals from the source node or from the other relay nodes with a fixed gain, and then forward the amplified information to the other relays or the destination. Such a relay technique is referred to as the “fixed-gain amplify-and-forward” technique. In the case when the received signals at the relays are amplified by a gain that depends on the instantaneous power of the channel, the transmission protocol is referred to as “variable-gain amplify-and-forward”. In other relay systems, the received signals are detected, re-encoded, and then forwarded at the relay nodes. Such a relay protocol is referred to as “decode-and-forward”. It is known that decode-and-forward yields a better performance than amplify-and-forward, while subject to a higher complexity at the relay nodes. In some application scenarios where simple relay nodes are desired, amplify-and-forward, especially fixed-gain amplify-and-forward techniques are more attractive.
With an ever increasing demand of information exchange over a huge number of nodes, M2M communications faces a number of challenges including how to select, among all possible transmission routes, the best route for signal transmission from the source to the destination. Intuitively, assuming channel state information (CSI) of all the transmission links is known at each node, an exhaustive search over all the possible routes can be performed to find the optimum route. However, this is not feasible in practice because of the prohibitive complexity introduced as the number of relays as the number of hops increases.